The general architecture of a typical high speed, broad bandwidth integrated optical modulator, having a single input, CPW-configured electrode structure overlying its interaction region, is diagrammatically shown in plan in FIG. 1, while FIG. 2 shows a typical optical modulator containing a single drive, coplanar strip-configured electrode structure overlying its interaction region. Except for these two drive electrode structures, the modulators are substantially the same.
Each comprises an electro-optic substrate 10 having a generally planar top surface 11, and a prescribed thickness, e.g., on the order of 1 mm. The substrate 10 may comprise a material such as, but not limited to, Z-cut or X-cut lithium niobate (LiNbO3), which is capable of supporting electro-optic modulation of an optical beam traveling through an interaction region, designated by dotted lines 30. An optical beam 13, such as that supplied by an input optical fiber and interfaced with the modulator by an associated fiber coupler (not shown), is coupled to an input port 21 of an optical waveguide 20, that is formed in the top surface 11 of substrate 10.
In such a modulator, optical phase or amplitude modulation of the optical beam results from an interaction between the optical beam 13, as it travels through the optical waveguide 20, and an RF microwave signal applied to a coplanar waveguide electrodes 50 formed on the surface of the substrate overlying the interaction region containing the optical waveguide. At its output or downstream end, a modulated output optical beam 15 is extracted from an output port 22 of the optical waveguide 20, by way of a fiber coupler (not shown).
As illustrated in the plan view of FIG. 3, the optical waveguide 20 may be configured as a Mach-Zehnder interferometer, having a pair of parallel, spaced apart, and generally rectilinear optical waveguide branch regions 23 and 24, that are joined together adjacent to the respective input and output ports 21 and 22, and lie within the electro-optic interaction section 30 of the modulator. Optical waveguide 20 is typically formed by selectively introducing (e.g, diffusing) a dopant, such as titanium, to a prescribed concentration and depth into the top surface 11 of the substrate, as is customary in the art.
Overlying the optical waveguide 20 is a primary buffer layer 40 of dielectric material, such as silicon dioxide or other suitable material. The primary buffer layer 40 is dimensioned to encompass at least the area of a coplanar RF energy coupling electrode structure 50, and is interfaced with associated RF microwave launch and termination interfaces 60 and 70. In the coplanar waveguide-configured electrode structure architecture of FIG. 1, RF energy coupling electrode structure 50 is configured as a coplanar waveguide, having a center ‘signal’ or ‘hot’ electrode layer 51, plus a pair of ground electrode layers 52 and 53 that extend along both sides of the signal electrode 51. In the coplanar strip-configured electrode structure of FIG. 2, the RF energy coupling electrode structure 50 is configured as a coplanar strip structure, having two parallel electrode layers 55 and 56. One electrode layer, e.g., electrode layer 55, serves as a signal electrode, while the other electrode layer, e.g., electrode 56, serves as the ground electrode. In each modulator, the electrodes are selectively formed atop the primary buffer layer 40.
Within the interaction section 30, the electrode layers of the coplanar electrode structure 50 are generally rectilinear and parallel to the underlying optical waveguide structure 20. At an input or launch region of the interaction section 30, the electrode layers of the coplanar electrode structure 50 extend to and are integrally coupled with associated electrodes of the RF microwave signal launch section 60. Similarly, at an output or termination region of the interaction section 30, the electrode layers of the coplanar electrode structure 50 extend to and are integrally coupled with associated electrodes of RF microwave signal termination or output section 70.
In this type of modulator architecture, bandwidth is limited by the phase matching of the optical and microwave traveling waves (as the two waves typically travel at different velocities, depending on the design of the device), by insertion (radio frequency) losses in the electrode structure, and by confinement of electric field energy of the dominant CPW mode and leakage of substrate modes of the microwave signal. Ideally, most of the microwave power is confined by the RF transmission line to only the surface of the substrate. In the electro-optic interaction region, the RF ‘hot’-to-ground gap is typically less than a distance on the order of twenty microns for a ten micron wide hot electrode. It has been observed that the electric field provided by this tightly confined electrode structure can effectively suppress coupling of RF energy into the substrate up to around 25 GHz.
In order to be mechanically and electrically effective to interface with an external RF transmission line, the width of the hot electrode of the launch interface is typically on the order of 250 microns. In addition, to match the characteristic line impedance of the interaction region (which is typically on the order of forty ohms), the gap of the launch hot electrode may be on the order of 350 microns. Because of this relatively wide ground-to-ground electrode spacing within the launch (e.g., on the order of 1 mm), coupling to substrate modes can be expected in the area of the launch (outside the interaction region) for an operational frequency above 25 GHz.
The conventional technique to suppress such coupling of RF energy to substrate modes in the launch area is to reduce the thickness of the substrate. A number of investigations have shown that a substrate thickness on the order of 0.25 mm is sufficient to suppress such unwanted coupling up to a frequency on the order of 40 GHz. When such substrate-thinning is carried out, the resulting, relatively fragile, wafer may strengthened by bonding to a 0.5 mm thick lithium wafer prior to dicing it into chips. Of concern is in-service reliability and crack propagation over temperature. Recent investigations and literature have reported further suggestions that extended-width ground electrodes result in higher insertion loss due to surface leakage modes and coupling of spurious substrate modes.